Various methods for determining the location of transmission sources exist: self-positioning methods, time of arrival (TOA) methods, and angle of arrival (AOA) methods. In self-positioning methods, a unit has a transmission source and a receiver, such as a Global Positioning Satellite (GPS) receiver or Iridium receiver, capable of determining the location of the mobile unit based upon information received by the receiver from, for example, a Global Positioning satellite. This information may be periodically uplinked (transmitted) to a basestation, where it is stored in a database until needed.
TOA methods are subject to a cooperation protocol, where several basestations each estimate their range (i.e., distance) to a selected mobile unit. These estimated ranges are forwarded to a data fusion processor, which substitutes these estimated ranges into a navigation equation to solve for the position of the mobile unit. One disadvantage of existing TOA methods is that a stable time reference typically must be provided. In systems that employ GPS, the time reference is provided by a series of distributed atomic standards, which are each expensive (e.g., greater than about $1,000) and heavy (e.g., greater than about five pounds). In some systems, the mobile unit includes a sophisticated transponder to enable round trip flight time measurements. These transponders can be expensive, costing about $50 when purchased in large quantities, and are not very small, having a form factor about the size of a pocket calculator. Accuracy in TOA systems may degrade substantially in indoor and urban environments due to multipath propagation. Some of this degradation may be recovered using subspace or time-frequency processing, at the cost of a substantial computational penalty relative to conventional correlation processing.
In AOA systems, basestations deploy an active antenna array and employ multichannel signal processing. Because the carrier phase observed at each array element has a unique relationship to the bearing of the transmission source, the array element outputs may be processed together to provide an estimate of the source direction as measured by each array element. These estimated directions are forwarded to a fusion processor, which solves the geometry for the source location based upon the estimated directions. Some disadvantages of AOA systems are that the basestation typically must be equipped with a smart antenna, including multiple antenna elements, a multichannel data acquisition system, and the basestation must perform substantial signal processing. In order to maintain bearing accuracy, an AOA system must be regularly calibrated to compensate for drift in system components. Both the initial expenditure for each of the hardware and software components and the need for calibration limit the applicability of AOA to systems that can absorb the associated cost penalty. Also, as in TOA systems, the accuracy of AOA degrades rapidly in multipath environments.
A general deficiency in prior art systems is that they do not effectively determine the location of a transmission source without the use of large, relatively expensive transmitter and receiver hardware. Further, prior art systems do not effectively function in multipath environments.